After an oil or gas well is drilled, it is often necessary to log the well with various measuring instruments. This is usually done with wireline logging tools lowered inside the well on a logging cable. Similarly, pipelines may require inspection and, therefore, the movement of various measuring tools along the pipe.
Some logging tools can operate properly only if they are positioned at the center of the well or pipe. This is usually done with centralizers. All centralizers operate on the same general principle. Equally spaced, multiple bow springs or linkages of various kinds are extended radially from a central hub toward the wellbore or pipe wall. These springs or linkages come into contact with the wellbore or pipe wall and exert radial forces on it which tend to move the body of the tool away from the wall. Since the bow springs and linkages are usually symmetric with respect to the central hub, they tend to position the tool at the center of the well. Hence, the radial forces exerted by these devices are often referred to as centralizing forces.
Centralizers usually remain open throughout their operation. In other words, their linkages are always biased toward the wellbore wall and they always remain in contact with the wellbore wall. Most centralizers are designed such that they can operate in a large range of wellbore sizes. As the centralizers expand or contract radially to accommodate changes in the size of the wellbore, their centralizing forces may vary. In wells that are nearly vertical, the variation in radial force is not a problem because the radial component of the tool weight is small and even weak centralizers can cope with it. In addition, the centralizing force and the frictional drag resulting from it are such a small fraction of the total tension on the logging cable that its variability can be neglected for all practical purposes.
Wells that have horizontal or highly deviated sections may, however, present problems. In a horizontal section of the well, the centralizer must be strong enough to lift the entire weight of the tool off the wellbore wall. On the one hand, the minimum level of the centralizing force must be made equal to the weight of the tool to ensure proper operation in all wellbore sizes. On the other hand, in a different wellbore size, the force exerted by the centralizer may be excessive, causing extra frictional drag that impairs the motion of the tools along the well. This situation has led to the desire for constant force centralizers, of which attempts have been made. However, current “constant force centralizers” do not produce a constant force, only a less variable force than previous attempts. Embodiments of the present invention, on the other hand, provide a truly constant force centralizer.
Similar to centralizers, calipers extend arms or linkages radially outwardly from the tool body toward the wellbore wall. One difference between centralizers and calipers is that the arms of a caliper may be individually activated and may not open the same amount. Another difference is that caliper arms are usually selectively opened and closed into the tool body by some mechanical means. Thus, the arms of a caliper do not necessarily remain in contact with the wellbore wall at all times.
Various measuring instruments are often mounted on the caliper arms. In order to ensure the proper operation of some of these measuring instruments, it is often necessary to maintain a certain range of the magnitude of the radial force with which the caliper arms are pressed toward the wellbore wall. This requirement is sometimes difficult to achieve in horizontal sections of the well and variable wellbore sizes. The reason is that, like centralizers, the mechanical advantage of caliper linkages varies with wellbore size. Thus, the mechanical devices responsible for opening and closing the caliper must provide variable force output. This usually leads to poor efficiency of the mechanical device and its under-utilization in a large range of wellbore sizes. It is, therefore, beneficial to develop caliper linkage mechanisms that apply virtually constant radial forces given a constant mechanical input from the actuation device. Embodiments of the present invention provide such a mechanism.
Horizontal and highly deviated wells present yet another problem. Logging tools cannot be effectively conveyed into such wells by the force of gravity. This has led to the development of alternative conveyance methods. One such method is based on the use of a downhole tractor that pulls or pushes logging tools along the well.
Downhole tractors, such as those described in U.S. Pat. Nos. 5,954,131 and 6,179,055 B1, use various radially expandable mechanisms to force wheels or anchoring devices against the wellbore wall. Independent of the principle by which the motion with respect to the wellbore wall is achieved, the traction force that a tractor can generate is directly proportional to the radial force applied by the mechanism. Similar to centralizers and calipers, downhole tractors are designed to operate in a wide range of wellbore sizes. Like centralizers, they also have the problem of radial force variability as a function of wellbore size. Typically, for a given expansion mechanism, the traction force diminishes with wellbore size. It is advantageous if the radial force that a tractor generates is constant. However, no satisfactory solution to this problem has thus far been disclosed.
Some tractors use several sets of different size linkages to provide a relatively constant traction force in a wide range of wellbore sizes. These mechanisms must, however, be replaced at the surface, which is very inconvenient. In addition, some wells are drilled with a variety of wellbore sizes that no single mechanism can handle. Embodiments of the present invention provide a mechanism that may be used with a tractoring device to achieve a constant radial force and, therefore, consistent traction over a very wide range of wellbore sizes.
Centralizers, calipers, and tractors all rely on radially expandable mechanisms to perform their functions. Accordingly, a need exists for a constant force actuator for use in centralizers, calipers, and tractors and other appropriate devices.